What is the size of the default grid floor?
Blender 2.79 or earlier
The X-Y grid that is visible in the viewport at all angles is 16x16 &quot;units&quot; large.

It is infinite if you are in any of the ortographic views: Top Numpad 7, Right Numpad 3 or Front Numpad 1.
What is a unit?
By default the &quot;units&quot; are dimensionless. But you can make them be meters or inches by selecting &quot;Metric&quot; or &quot;Imperial&quot; from the &quot;Length&quot; menu under the Scene panel


Blender 2.8 Alpha
In Blender 2.8 Alpha the base grid is infinite. The units are Meters by default.